While the detailed mechanisms of tumorigenesis are unknown, increasing evidence suggests that genetic alterations of cellular oncogenes are, in part, responsible for the neoplastic transformation of cells. Some studies in rodent models have implicated the direct chemical activation of oncogenes by carcinogen exposure. The reproducible detection of specific transforming genes in animal model systems strongly suggests that these genes have a significant role in the development of certain tumors. Data from our preliminary transfection experiments with DNA from carcinogen-induced medaka tumors supported this hypothesis and suggested that an unknown gene may be activated in the DEN-induced cholangiocarcinoma. It does not appear to have homology to any known oncogene sequence based on hybridization of Southern blots. Recently, studies have been initiated to examine DNA from gonadal tumors in two species of clams (mya arcnaria and mercenatia mercenara) exposed to herbicides. We have demonstrated transformation of NIH3T3 cells and have produced one tumor in nude mice using DNA from an advanced tumor. The identification and characterization of this apparently novel oncogene is now in progress. Transgenic medaka were produced by the introduction of the E. coli lacZ gene under the control of the mouse Mt-I metallothionein promoter. Fish which contain the lacZ gene have been identified by PCR analysis. This construct has also been introduced into fish cell lines to form the basis of in vitro toxicity assays. We have not yet been able to demonstrate expression of this construct in either fry or the fish cell lines.